1. Field of the Invention
The present invention generally relates to an engine exhaust gas cleaning apparatus. More specifically, the present invention relates to an improvement to exhaust gas cleaning apparatuses for engines equipped with a NOx trapping catalytic converter in the exhaust system.
2. Background Information
In internal combustion engines such as diesel engines, an exhaust gas recirculating system (EGR system) is widely used wherein a part of the exhaust gas is recirculated to lower the combustion temperature in order to reduce discharge of nitrogen oxide (NOx). Some prior exhaust systems for diesel engines often include a diesel particulate filter (DPF) and a NOx trapping catalytic converter that are disposed in an exhaust passage. The diesel particulate filter (DPF) traps particulate matters (PM) in the exhaust gas, while the NOx trapping catalytic converter traps NOx in the exhaust gas when an air-fuel ratio in the exhaust gas is in a lean range and purifies the trapped NOx when the air-fuel ratio is in a rich range.
Thus, an engine having a NOx trapping catalytic converter normally operates with lean combustion and the NOx generated during that operation are captured in a NOx absorbing agent. When a certain amount of NOx has accumulated, the excess air ratio λ of the exhaust gas is temporarily changed to a rich value to desorb and deoxidize the NOx from the absorbing agent. This control of the excess air ratio λ of exhaust gas being recirculated that temporarily shifts the value of the excess air ratio λ to a rich value is called “rich spike control.”
Diesel engines are normally operated with an excess air ratio λ of approximately 2 to 3, which corresponds to a lean air fuel ratio. During rich spike control, the value of the excess air ratio λ is changed to approximately 0.8. Rich spike control is normally executed with non-regular timing in response to engine operating conditions to treat the NOx accumulated in the NOx trapping catalytic converter. In other words, rich spike control is not executed in response to a request that is issued by the driver. Consequently, the driver will experience a feeling of discomfort if the torque fluctuates during the shift from the normal operating state to the rich spike control state. Therefore, in order to reduce the value of the excess air ratio λ while changing the fuel delivery quantity as little as possible, the rich spike control is designed to reduce the quantity of intake air by reducing the opening degree of a throttle valve provided in an intake passage, as disclosed in Japanese Laid-Open Patent Publication No. 2003-129890.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved engine exhaust gas cleaning apparatus. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.